Contact lenses having a toric optical zone (commonly referred to as “toric contact lenses”) are used to correct refractive abnormalities of the eye associated with astigmatism. The toric optical zone provides cylindrical correction to compensate for astigmatism. Since astigmatism that requires vision correction is usually associated with other refractive abnormalities, such as myopia (nearsightedness) or hypermetropia (farsightedness), toric contact lenses are generally prescribed also with a spherical correction to correct myopic astigmatism or hypermetropic astigmatism. A toric surface is formed on either the posterior lens surface (to achieve a “back surface toric lens”) or the anterior lens surface (to form a “front surface toric lens”).
Whereas spherical contact lenses may freely rotate on the eye, toric contact lenses have a stabilization structure to inhibit rotation of the lens on the eye so that the cylindrical axis of the toric zone remains generally aligned with the axis of the astigmatism. In general, a stabilization structure may include one or more sections of the lens periphery which are thicker (or thinner) than other sections to provide the stabilization. For example, a ballast is a thick portion that may assume a downward position when a lens is inserted in an eye, and the ballast axis of a lens extends in the vertical direction when a lens assumes its position on the eye. Other stabilization structures are known in the art. Regardless of the structure of the stabilization structure, the axis that extends in the up and down direction when a lens assumes its position on the eye will be referred to herein as the ballast axis.
The nature of prism ballasting is to move the apex of the lens down from the horizontal meridian. In traditional prism ballasted lenses, a crescent shaped portion of the overall larger anterior optical zone is located outside of the posterior optical zone. This crescent shaped portion of the anterior optical zone is the most inferior portion of the optical zone and thickens thereby resulting in decreased Dk/t and comfort.
Toric contact lenses are manufactured with a selected relationship (referred to herein as offset) between the cylindrical axis of the toric optical zone and the orientation of the stabilization structure. This relationship is expressed as a number of degrees that the cylindrical axis is offset from the ballast meridian. Tonic contact lenses are also manufactured with a selected orientation of the sphere power meridian of the toric surface relative to a horizontal meridian as determined by a corresponding rotational stabilization structure (e.g., a contact lens prism ballast). The orientation is referred to herein as an angular offset (hereinafter referred to simply as axis). For example, this relationship may be expressed as a number of degrees that the sphere power meridian is angularly displaced from a horizontal meridian of the lens as determined by the ballast. Accordingly, toric contact lens prescriptions specify offset, with toric lenses generally being offered in 10-degree increments ranging from 10 degrees to 180 degrees.
In toric contact lenses, the optical zone provides cylindrical correction to compensate for astigmatism. The resulting optical zone has a sphere power meridian and a cylinder power meridian. The orientation of each of the above meridians is best understood with reference to conventional contact lens prescriptions. In a prescription −3.00/−1.25, the sphere power meridian is the meridian having a power equal to −3.00 diopters and the cylinder power meridian is the meridian having a power equal to −4.25 diopters. In a prescription +3.00/−1.25, the sphere power meridian is the meridian having a power equal to +3.00 diopters and the cylinder power meridian is the meridian having a power equal to +1.75 diopters.
Rotational stability may be achieved using a number of ballasting designs including inferior truncation, double truncation, thin zones (also called “double slab-off”) and prism wedge profile. These ballasting designs may be used individually or in combination. One common feature of these ballasting designs is the use of different thickness areas in the lens to achieve rotational stability. For example, in the case of a “slab off” design, the top and bottom portions of the lens are thinned out such that when they are positioned under the eyelids they are held in place by the lids. At the same time, the thicker portions of the lens are positioned in between the eyelids where they too are held in place by abutting against the eyelids.
Although entirely satisfactory in many respects, there is still a need for additional types of ballasting designs for a toric lens. This need is especially seen in efforts to provide toric lenses having better optics, improved comfort and better on eye orientation in both rotational recovery and primary gaze orientation (PGO) than traditionally ballasted lenses.